Today, it is important to consumers that portable devices have a long battery life. Many portable devices include GPS receivers that enable the consumers to navigate from one place to another. GPS acquisition performance is typically limited by four items of information:                a) The accuracy of a reference frequency on a GPS receiver        b) The accuracy of the GPS receiver time relative to GPS system time        c) Availability of currently valid Ephemeris information (high precision satellite orbital parameters.)        d) The accuracy of an initial position estimate of the GPS receiver        
GPS navigation devices consume a fair amount of power to realize high speed GPS acquisition performance, resulting in shorter battery life for GPS devices. Currently, the available techniques for improving the above items a) and b) are limited with respect to power consumption.
For example, the reference frequency often comes from a temperature compensated crystal oscillator (TCXO) in most GPS receivers. The TCXO typically runs when the GPS receiver is running and not when the GPS receiver hibernates because the TCXO can use too much power if it is left always on. It is a much more accurate frequency reference than a real time clock (RTC) crystal oscillator. But since the TCXO cannot be left always on, the TCXO cannot be used for a time reference, because when the power is off, a time counter using that reference frequency would stop. The RTC crystal oscillator is typically used as a time reference for the above reasons, among others.
Currently, the accuracy of time from the RTC crystal oscillator is typically limited by unknown frequency excursions, among other factors. Such unknown frequency excursions are typically dominated by temperature changes at the RTC crystal oscillator, during the time when the RTC crystal oscillator cannot otherwise be calibrated to a more accurate time reference because the GPS is shut off (in hibernate mode) to save battery power. If an external source of time is available, such as from a radio network, then the time accuracy is limited to the accuracy of the network source. Sometimes the network time is not available, or not very accurate for the purpose of this application.
The knowledge of the reference frequency is typically limited by the accuracy and environmental (temperature) stability of the reference TCXO. Frequency can also be derived from some network radio receivers, but this may not be quick, easy to implement, accurate, or even available.